An environmental concern in regard to air pollution pertains to waste gas streams containing nitrogen oxides (hereinafter referred to as NO.sub.x) and sulfur oxides (hereinafter referred to as SO.sub.x). Nitrogen oxides consist primarily of nitric oxide (NO) and nitrogen dioxide (NO.sub.2), while sulfur oxides consist primarily of sulfur dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3).Such streams are produced, e.g. as emissions from power generating plants, as the result of combustion of fuels containing nitrogen and sulfur compounds. Also, during the manufacture of certain chemical products, such as nitric acid and organic nitrates, effluent streams containing NO.sub.x are produced. Further, NO.sub.x are also produced by the oxidation of atmospheric nitrogen by atmospheric oxygen at the high temperatures which exist in the course of fossil fuel combustion.
The NO.sub.x and SO.sub.x are desirably removed from waste gas streams prior to discharge of the streams into the atmosphere because NO.sub.x and SO.sub.x can cause respiratory ailments such as emphysema and further can oxidize and combine with atmospheric water vapor to form oxyacids of nitrogen and sulfur. Such acids then fall to the earth as "acid precipitation" undesirably making the environment more acidic. Moreover, it is well known that NO.sub.x additionally contributes to air pollution by taking part in the formation of photochemical smog.
While it is possible to avoid or at least minimize emissions of SO.sub.x and, to a lesser extent, emissions of NO.sub.x since much of the NO.sub.x is produced from the atmospheric components of air, by using clean fuels such as light fuel oil or natural gas, such fuels are relatively expensive. It would be most desirable if methods could be employed to remove the NO.sub.x and SO.sub.x produced using cheaper power-generating fuels such as coal and heavy fuel oil using conventional types of flue gas desulfurization units.
Any method used to remove NO.sub.x and SO.sub.x from waste gas streams must not only result in reduction of the NO.sub.x and SO.sub.x to acceptable levels, but must also be cost effective from the standpoint of capital costs as well as operating costs. Processes are known for removal of NO.sub.x and SO.sub.x from waste gas streams which have low operating costs in the sense that relatively cheap chemicals are used for sorbing the NO.sub.x and SO.sub.x. As a general rule, however, such processes entail relatively high capital costs.
Coal may be combusted to generate power and the resultant flue gas stream may be scrubbed with an aqueous alkaline solution to sorb the SO.sub.x present in the stream. However, such solution will have little effect on reducing the NO.sub.x present in the stream to acceptable levels. As a result, the NO.sub.x must be reduced to acceptable levels by combustion control equipment which is relatively expensive. Moreover, controlling the combustion parameters so as to minimize the formation of NO.sub.x in the waste gas stream frequently results in a diminution of the maximum combustion efficiency from the standpoint of power generation.
It would be desirable if a process could be provided which permits conventional aqueous alkaline solution scrubbing units to be utilized for removal of NO.sub.x from waste gas streams without requiring any expensive modifications to such units and without affecting the capability of the units to quantitatively absorb the SO.sub.x present in the waste gas stream. It would be most desirable from the perspective of maintaining low operating costs if any additive used in such units for sorbing the NO.sub.x was not parasitically consumed by the presence of the SO.sub.x present in the waste gas stream, i.e. the SO.sub.x present in the stream would quantitatively dissolve in the liquid scrubbing solution and would not react with the additive used for sorbing the NO.sub.x present in the stream.